Movements in the dark : flying, landing and walking in insects

Abstract: Flying, as well as walking insects rely on vision to regulate locomotion, even in the dark when the visual system is much less reliable. To manage visual control of these behaviours at low light intensities, many insects have evolved optical adaptations, such as larger facet lenses and wider rhabdoms, and neural adaptations, such as spatial and temporal summation, to increase their visual sensitivity.To investigate the effect of light intensity on flight control in crepuscular insects, I filmed bumblebees flying through an experimental tunnel at different light intensities. I found that bumblebees control their flight well even in dim light but fly slower as light levels fall. We also measured the effect of light intensity on the response speed of bee photoreceptors and found that they respond more slowly at lower light intensities. These results indicate that bumblebees compensate both behaviourally and visually to be able to fly in dim light.Next, I examined the final moments of landing in bumblebees by training them to land on a flat platform that could be rotated to different orientations. I found that bumblebees adjust their body and head posture depending upon the orientation of the platform and that leg extension occurred at a constant distance from the surface (except at low platform tilts). I also investigated the effect of light intensity on the landing precision in bumblebees while landing at the same platform at two different orientations and at different light intensities. I found that bumblebees perform well-controlled landings in dim light, however, as light intensity decreased, the bees oriented their body more vertically and their head more horizontally relative to the horizontal plane and extended their legs further away from the platform. These results indicate that bumblebees rely on visual cues to perform smooth landings even in dim light.Finally, to investigate how walking insects adapt to dim light, we analysed the orientation performance of diurnal and nocturnal dung beetles while rolling their dung balls from the centre to the periphery of a circular arena in the lab as well as in the field. We found that both species oriented well to a point light source, such as the moon or an artificial light. When only wide-field cues were present, such as starlight or the polarization pattern around the moon, the nocturnal beetles were much better oriented. Moreover, we found no effect of light intensity on ball-rolling speed, suggesting that these beetles do not employ temporal summation strategies, but rather a spatial summation approach to adapt to dim light.To summarize, the data presented in this thesis has broadened our knowledge about insect flight, landing, walking and orientation performance in dim light and has given insights into which adaptations they might use to meet the challenges of unreliable visual signals.